Proportioning Valve
there's nothing to them ......
take it apart, clean it, lube it with silicon O-ring lube, reassemble and done.
If it won't go back together, it's probably because the o-rings are swollen. You can't fix that... or at least I don't know how to .... so I replace them. The round ones are easy to find. But if you have a specialty ring in there, I just grab another valve from the used parts pile, lol, and start over.
As for testing it IDK ...... I test it on the car, lol, after I rebuild it.